Process for bleaching preoxidized paper pulp

Abstract

The invention relates to a process for bleaching preoxidized paper pulp enabling a pulp having a very high degree of whiteness to be obtained with a limited number of treatment steps and enabling pollution to be reduced without degrading the qualities of the pulp; such a process essentially comprises the combination of the three successive treatment steps, namely: (a) a first step of hot chlorination with gaseous chlorine at a temperature of between about 30 DEG and 80 DEG C., and preferably between 35 DEG and 50 DEG C., in the presence of urea used at a rate of 0.5 to 5% by weight of the chlorine used, and preferably from 1 to 3%, with a concentration of pulp of between 3 and 30%, and a duration of retention of the chlorine of 30 seconds to 45 minutes; (b) a second step of alkaline extraction in the presence of an oxidizing agent of the hypochlorite or peroxide type, at a temperature of between about 30 DEG and 70 DEG C., and at a pH of between about 8 and 12, and preferably between 10.5 and 11.5, and (c) a third step of treatment with chlorine dioxide, at a temperature of between 60 DEG and 90 DEG C. and preferably between 70 DEG and 80 DEG C., at an initial pH of between 5 and 11, and preferably between 7 and 9, a recycling of the effluents of the second and/or the third treatment step for intermediate washing preceding the corresponding step being provided.

What is claimed is: 
     
       1. A process for bleaching an oxygen or peroxide preoxidized paper pulp to produce a pulp having a very high degree of whiteness with a limited number of treatment steps while reducing pollution without degrading the qualities of the pulp comprising essentially the combination of three successive treatment steps, namely: (a) a first step of hot chlorination with gaseous chlorine, at a temperature of between about 30° and 80° l C. in the presence of urea in an amount between about 0.5 to 5% by weight of the chlorine used, with a concentration of pulp between 3 and 30%, and a duration of retention of the chlorine of 30 seconds to 45 minutes;   (b) a second step of alkaline extraction in the presence of an oxidizing agent of the hypochlorite type, and in the presence of urea in an amount between about 1 to 10% by weight of the chlorine in the form of hypochlorite, at a temperature of between about 30° and 70° C. and a pH of between about 8 and 12; and   (c) a third step of treatment with chlorine dioxide, at a temperature of between about 60° and 90° C. at an initial pH of between about 5 and 11, and recycling effluent of at least one of second and third treatment steps for intermediate washing preceding the corresponding step.   
     
     
       2. The process as in claim 1 wherein the concentration of pulp in the first chlorination step is between about 8 and 15% and the duration of retention of the chlorine is about 30 seconds to 5 minutes. 
     
     
       3. The process as in claim 1 wherein effluent from the second bleaching step is recycled in order to neutralize the pulp issuing from the first step of bleaching. 
     
     
       4. The process as in claim 1 wherein effluent from the third step of bleaching is recycled for washing the pulp issuing from the second step to control the initial pH of the third step. 
     
     
       5. The process as in claim 1 in which said first step is carried out at a temperature between about 35° and 50° C. 
     
     
       6. The process as in claim 1 in which the amount of said urea in the first step is between about 1 to 3%. 
     
     
       7. The process as in claim 1 in which said second step is carried out at a pH of about 10.5 to 11.5. 
     
     
       8. The process as in claim 1 in which said third step is carried out at a pH between 7 and 9. 
     
     
       9. The process as in claim 1 in which said first step is carried out at a temperature of about 35° to 50° C. and in which the amount of urea in said first step is about 1 to 3% of the amount of chlorine used. 
     
     
       10. The process as in claim 1 in which said second step is carried out at a pH between about 10.5 and 11.5 and in which said third step is carried out at a pH between about 7 and 9. 
     
     
       11. The process as in claim 1 in which said third step is carried out at a temperature of about 70° to 80° C. 
     
     
       12. The process as in claim 2 in which said first chlorination step, the concentration of the pulp is between 9 and 11%.